docs(raftengine): PR783 r2 claude nits — fix stale struct-comment + cleaner errors.Newf wrap

Claude r2 on PR #783 flagged two cosmetic items:

## Issue A (blocking per "before merge") — stale struct comment

The encryptionScanner struct comment said "constructed once per
Engine and embedded into the engine struct" but
Engine.EncryptionScanner() allocates a fresh value on each call
(intentionally — the scanner is stateless). Updated to "wraps the
engine's *MemoryStorage; a fresh value is constructed per
Engine.EncryptionScanner() call (cheap — no internal state)" to
match the implementation.

## Issue B (nit) — awkward errors.Wrapf(errors.New(...))

The no-progress fail-closed path used
errors.Wrapf(errors.New("no progress"), "encryption scanner: ...")
which reads as "encryption scanner: ...: no progress" — making
the sentinel look like the cause rather than just the
description. There's no caller that needs to errors.Is on the
"no progress" sentinel, so collapsing to
errors.WithStack(errors.Newf(...)) is cleaner. Matches the
existing pattern at internal/admin/config.go:109+ for newly-
constructed errors that satisfy wrapcheck.

## Verification

- go test -race -timeout=60s ./internal/raftengine/etcd/... — PASS
- golangci-lint run ./internal/raftengine/etcd/... — 0 issues

Author: bootjp

internal/raftengine/etcd/encryption_scanner.go

@@ -21,9 +21,10 @@ import (
 const scanMaxBytes uint64 = 64 << 20 // 64 MiB
 
 // encryptionScanner implements encryption.EncryptionRelevantScanner
-// against the engine's in-memory raft storage. It is constructed
-// once per Engine and embedded into the engine struct; callers get
-// access to it via Engine.EncryptionScanner().
+// against the engine's in-memory raft storage. It wraps the
+// engine's *MemoryStorage; a fresh value is constructed per
+// Engine.EncryptionScanner() call (cheap — no internal state).
+// Callers get access to it via Engine.EncryptionScanner().
 //
 // The scan walks raftpb.Entry rows in (startExclusive, endInclusive]
 // via etcdraft.MemoryStorage.Entries(lo, hi, maxSize), filters out
@@ -85,9 +86,9 @@ func (s *encryptionScanner) HasEncryptionRelevantEntryInRange(startExclusive, en
 			// we surface it as a scanner error. The caller routes
 			// scanner errors as a refusal distinct from
 			// ErrSidecarBehindRaftLog.
-			return false, errors.Wrapf(errors.New("no progress"),
+			return false, errors.WithStack(errors.Newf(
 				"encryption scanner: Entries(cursor=%d, hi=%d) returned no entries and no error (unscanned gap)",
-				cursor, hi)
+				cursor, hi))
 		}
 		for _, ent := range batch {
 			if isEncryptionRelevantEntry(ent) {